Permafrost Thermal Dynamics in CMIP5 Earth System Models

Koven et al. (2013) write that "permafrost is a critical component of high-latitude land and determines the character of the hydrology, ecology, and biogeochemistry of the region." And, therefore, they say there is "widespread interest in the use of coupled atmosphere-ocean-land surface models to predict the fate of permafrost over the next centuries because 1) permafrost contains the largest organic carbon (C) reservoir in the terrestrial system (Tarnocai et al., 2009), 2) permafrost stability is primarily dependent on temperature, and 3) global warming is expected to be relatively larger over the permafrost domain because of arctic amplification processes (Holland and Bitz, 2003)." Recognizing this interest, Koven et al. analyzed "output from a set of Earth system models (ESMs) that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5) (Taylor et al., 2009) to evaluate the permafrost model predictions against observations and theoretical expectations and to compare the predicted fate of permafrost under warming scenarios."

In doing so, the three U.S. researchers determined that "the models show a wide range of behaviors under the current climate, with many failing to agree with fundamental aspects of the observed soil thermal regime at high latitudes." More specifically, they report that (1) "under future climate change, the models differ in their degree of warming, both globally and at high latitudes, and also in the response of permafrost to this warming," that (2) "there is a wide range of possible magnitudes in their responses, from 6% to 29% permafrost loss per 1°C high-latitude warming," that (3) several of the models predict that substantial permafrost degradation has already occurred (ranging from 3% gain to 49% loss relative to 1850 conditions)," that (4) "the majority of models at the high end of relative twentieth-century permafrost loss also show unrealistically small preindustrial permafrost extent," that (5) "there is wide model disagreement on the value of the difference in mean temperature across the air-soil interface, with several of the models [even] predicting the wrong sign for this statistic [italics added]," and that (6) "there is wide model disagreement in the changes of [the] mean and [the] amplitude of soil temperatures with depth."

In commenting on their findings, Koven et al. conclude by stating that "with this analysis, we show that widespread disagreement exists among this generation of ESMs," which once again suggests that current earth system models are not yet ready for real-world application.